We have previously shown that human angle judgement is equally accurately whether angles are static or moving (up to moderate speeds). Are the computational strategies for static and dynamic conditions the same?

We investigated angle discrimination for static and rotating (angular speeds from 90 /s to 360 /s) triangles defined by three dots (8 cpd D4s). Good performance with moving dot triangles presented on a background of identical static dots argues against the possibility that static ‘snapshots’ are used to discriminate moving angles. If a single frame is not used, over what duration of a movie sequence does the visual system integrate information?

To investigate this, the display was modified so that only two of the three dots were visible on each frame. Blank periods were introduced as transitions between dot pairs. Angle discrimination thresholds show a strong dependence on blank duration when triangles rotate, with an additional dependence on speed of rotation, but are much more resistant to blanks when they are static. For moderate rotational speeds, thresholds double for a 50ms blank, while a 250ms blank is required before thresholds double for static triangles.

In a second experiment, the role of the information provided before and after a transition between dot pairs was investigated by changing pair lifetimes. Surprisingly, unlike static angles, increasing lifetimes (from 100 to 170ms) decreases performance. This implies that factors such as the total number of transitions, and the time between them are important.

In summary, high sensitivity on dynamic shape discrimination cannot be explained by static ‘snapshots’. Moreover, blank and lifetime experiments show two ways in which dynamic performance differs from static performance.